According to a known automatic transmission apparatus, a rotation force generated by an engine is transmitted into the automatic transmission apparatus through a torque converter, and outputs a specified rotation speed obtained by converting the rotation speed of the input shaft, then the rotation force is transmitted to a driving wheel. The automatic transmission apparatus has a gear unit being a combination of gears of many sizes, and friction engaging elements such as a clutch and a brake are engaged for outputting a rotation of the selected gear. In this way, multiple rotation speeds can be shifted and transmitted through the automatic transmission apparatus.
The automatic transmission apparatus includes an oil pan provided under an automatic transmission case for storing oil used for cooling down and lubing rotating members such as the gears and a bearing. The oil stored in the oil pan is pumped by an oil pump through a strainer and provided to each rotating member. When each rotation member is rotated, the oil provided to each rotation member flies in all directions within the automatic transmission case in response to the centrifugal force of the rotating members. Then, the oil runs down due to gravitation and returns into the oil pan.
According to a forward and rear drive switching device of the known automatic transmission apparatus, a forward drive clutch drum is provided integrally with the input shaft, and a hub is provided integrally with a sun gear. The drum-shaped hub includes a cylindrical portion at the outer peripheral portion thereof extending in a direction away from the sun gear. The cylindrical portion includes a spline at an outer peripheral surface thereof, and the forward drive clutch drum is engaged with the spline (Refer to FIG. 5). An oil pass runs through from an inner peripheral portion of the sun gear to an outer peripheral portion of the sun gear, and one end of the oil pass on the sun gear's outer peripheral portion may be positioned between the gear portion and a hub connecting portion of the sun gear. (e.g. Japanese Patent Application Laid-Open Publication as H6 (1994)-221400, FIG. 1)
In the aforementioned forward and rear drive switching device of the known automatic transmission apparatus, however, the oil will remain inside the drum-shaped hub. In other words, the oil provided through the oil pass of the sun gear for lubing the forward driving clutch is spread to the inner side of the hub as well as the back side of the hub. The oil flow at inner of the hub may be prevented due to the engagement between the hub and the forward driving clutch drum, so that the oil tend to remain inside of the hub.
When the oil remains in the hub, problems may be occurred. If the capacity of the oil pan is not enough, the small amount of the oil is used for the automatic transmission apparatus. In this configuration, after a while the engine is started, the oil pump sucks out the oil from the oil pan, and then as the amount of the oil in the oil pan is decreased, the oil pump may suck the air. When the oil pump sucks the air, following problems may be occurred. Firstly, the oil pump will make a noise. Secondly, if the oil mixed with the air flows into a valve body provided at the downstream of the oil pump, an oil pressure might be changed, as a result a shift quality may be declined. Finally, a required oil pressure is not secured with the oil mixed with the air (refer to FIG. 6), and the engaging force of the clutch becomes weak, as a result, the clutch plates of the clutch may slide and burn.
To avoid aforementioned problems, the oil amount might be increased, however, if high-temperature friction members are soaked in the oil, the oil increases in temperature and deteriorates quickly. In addition, it becomes difficult to balance the friction qualities on both of the low-temperature side and the high-temperature side.
When the capacity of the oil pan is enough, sufficient amount of oil can be used and stored in the oil pan, so that the oil returned from the transmission apparatus side into the oil pan before the oil in the oil pan runs out. In this configuration, the aforementioned problems can be prevented, however, when the automatic transmission apparatus is provided, onto the a vehicle such as a truck whose loading space is provided on a chassis thereof and vehicle height is limited, the capacity of the oil pan is limited, as a result, the aforementioned problems can not be avoided.
Furthermore, when the vehicle is traveling, the output member such as an output shaft and a planetary gear are also rotated, so that the oil is smoothly returned from the transmission apparatus side, and the aforementioned problems can be avoided. On the other hand, when the vehicle is in an idling condition, the oil is not returned smoothly from the transmission apparatus side because the output member of the automatic transmission apparatus is not rotating, so that the aforementioned problems cannot be avoided. When the engine just starts, and the oil temperature is low, an oil's flow resistance increases, as a result, the oil is not flied even if the centrifugal force is applied, and the oil may not run down and may remain on the surface of the each rotating member. Such oil is not returned to the oil pan, and the oil amount returned to the oil pan is decreased, finally the oil level in the oil pan becomes lower.
The present invention therefore seeks to provide an automatic transmission apparatus in which the oil is returned into the oil pan as much as possible even if a capacity of the oil pan is not enough. The present invention also seeks to provide an automatic transmission apparatus in which the oil is returned into the oil pan as much as possible when the vehicle is in the idling condition. The present invention further seeks to provide an automatic transmission apparatus in which the oil is returned into the oil pan as much as possible when the oil temperature is low.